A prospective study of neonates with persistent pulmonary hypertension (PPHN) to determine optimal dosage of nitric oxide (NO) for improving oxygenation. This investigation is undertaken to determine the hemodynamic, ventilation, and oxygenation dose-response of inhaled NO in infants with PPHN, to characterize the hemodynamic, ventilation and oxygenation response to withdrawal of inhaled NO, and to identify the characteristics of responders vs non-responders. This knowledge will increase current understanding of PPHN and determine the mechanism of improved oxygenation with inhaled NO. The investigators hypothesize that optimal use of inhaled NO, will result in decreased toxicity, thereby decreasing morbidity and mortality of infants with PPHN.